1. Field of the Invention
The present invention relates to a zoom lens that is suitably used as a taking lens in an image pickup apparatus such as a video camera, a digital still camera, a broadcasting camera, a film camera and the like.
2. Description of the Related Art
There is a need for a zoom lens that is compact in size and has a high zoom ratio for use as a taking optical system in an image pickup apparatus using a solid state image pickup element, such as a video camera, a digital still camera and a film camera using a silver halide film.
As a zoom lens that meet such needs, a four-unit zoom lens having a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power arranged in the mentioned order from the object side to the image side has been known.
Some known four-unit zoom lenses having the above described configuration are what is called rear-focus zoom lenses in which the second lens unit is moved to change magnification and the fourth lens unit is moved to perform focusing and correction of image plane variations (see Japanese Patent Publication 2002-182109, Japanese Patent Application Laid-Open No. 2002-287027, U.S. Pat. No. 6, 118,593 and Japanese patent Application Laid-Open No. 2000-227548).
In recent years, it is highly desired that zoom lenses used in image pickup apparatuses have a high zoom ratio and be small in the size of the entire lens system.
In a zoom lens, in general, increasing the refractive power of the lens units that constitute the zoom lens leads to a decrease in the movement amount of the magnification changing lens unit(s) in achieving a predetermined zoom ratio. Thus, it is possible to reduce the entire length of the lens system and increase the zoom ratio by increasing the refractive power of the lens units.
However, if the refractive power of each lens unit is simply increased, variations of aberrations during zooming become so large that it becomes difficult to achieve excellent optical performance throughout the entire zoom range, while making the zoom ratio higher.
On the other hand, use of the rear focus system leads to a reduction in the size of the entire lens system, enables high speed focusing and makes close-up shooting easy.
On the other hand, however, use of the rear focus system leads to an increase in aberration variations during focusing, and makes it very difficult to achieve excellent optical performance throughout the entire object distance range from a infinite object distance to a short object distance.
When the zoom ratio is to be made higher, it is difficult achieve excellent optical performance throughout the entire zoom range
In view of the above, to achieve an increase in the zoom ratio and a reduction in the size of the entire lens system, it is important to appropriately design the zoom type, the refractive powers of the lens units, and the lens configurations of the lens units.